This invention relates to an optical data recording medium having an organic dye as the recording material and, more particularly to such a recording medium which has improved reflectivity.
Recently, studies have been conducted on the optical data recording system in the field of data recording. This optical data recording system has a number of advantages in that recording and reproduction may be made in a contact-free mode, a recording density can be made which is higher by more than one digit order as compared with the magnetic recording system may be achieved and the system adapts to memories which are dedicated to reproduction, such as write once type memories or rewritable type memories. Thus the optical data recording system is employed not only in industrial applications but in consumer equipment which enables a large capacity file to be realized at lower costs.
With the write once type memory, data may be recorded on and reproduced from the memory by the ultimate user. Since the data, once written, cannot be erased, this type of memory is mainly used as a file for long-term data storage. For recording, local irreversible physical changes of the recording layer, brought about by the absorption of the light energy by the recording material, followed by conversion of the absorbed energy into thermal energy, is utilized. These irreversible physical changes may result in changes in shape, caused by formation of pits, changes in surface properties and changes in the crystal state of the recording layer.
The recording materials employed in the presently commercialized write once type optical data recording medium are mostly tellurium or tellurium alloys. However, organic dyes are recently attracting attention in place of these tellurium materials because profitability and production of the recording medium may be improved. The organic dyes are required to exhibit conspicuous absorption in the near-infrared region which is the wavelength region of the semiconductor laser employed for recording and reproduction. Up to now, methine-base dyes, benzenedithiol nickel complexes, metal phthalocyanin dyes or naphthoquinone dyes have been known and have been used as the organic dyes.
In order for desirable recording and/or reproducing characteristics to be achieved in the optical data recording medium, the following conditions need to be satisfied in connection with the physical properties of the recording materials. For achieving high recording sensitivity, it is necessary that the rate of light absorptivity be high, that the thermal capacity and conductivity be low and that thermal changes for recording occur at lower temperatures. On the other hand, for achieving high reproduction sensitivity, it is necessary that changes in reflectivity before and after recording be high, that the shape of the formed pits be smooth and that a small amount of noise be produced. Among the above desirable conditions, high absorptivity and high reflectivity are the most fundamental optical properties.
However, the organic dyestuffs which have so far been known exhibit only low reflectivity of 30 to 40 percent at most, when formed into a film, such that a sufficient reproduction sensitivity cannot be realized. When it is attempted to reproduce an optical data recording medium, formed of organic dyestuffs as the recording material, using a reproducing apparatus adapted for reproducing an optical disk comprised of a substrate having pits formed thereon and an aluminum reflective film formed on the substrate, or so-called compact disk, it is necessary for the reflectivity to be at least at high as that of the currently commercialized compact disk, which is 70 percent or more at 780 nm. However, in a manner different from the compact disk, which is dedicated to reproduction, the write once type optical data recording medium has a drawback in that the reflectivity is improved at the cost of the recording sensitivity. The reason for this is, if a high reflectivity is to be achieved at a certain wavelength, the rate of light absorption at such wavelength needs to be low, however, the irreversible physical changes cannot be brought about effectively if the light absorptivity is lowered. In order to solve this problem, organic dyestuffs need to be found in which the amount of the absorbed light energy, even if small, may be converted effectively into thermal energy so as to bring about the irreversible physical changes.
It is extremely difficult to find such dyes, among the above enumerated organic dyes, which exhibit satisfactory properties for the recording material, such as resistance to light degradation, has high weatherability and has excellent solubility in general-purpose solvents.
On the other hand, with cyanine dye, which is attracting attention as the recording material, since the laser wavelength falls on the skirts of the absorption region of the dyestuff material, the absorption and hence the reflectivity tend to be changed significantly with only small changes in wavelength. Cyanine dye, for example, exhibits reflectivity at 780 nm which differs by about 10% from that at 770 nm. Since the allowable wavelength range of a laser diode is about 780+10 nm, it is difficult to obtain the same reflectivity for all of the laser diodes, and thus the signal intensity may drastically differ from one reproducing unit to another.